Receiving system for code signals



Aug. 23, 1949. B. s. VILKOMERSON 2,479,652

RECEIVING SYSTEM FOR CODE SIGNALS Filed Jan. ll, 1945 uuunn A YAA u N w' C: u l `I t L E5 gm t l i f" TTR/VEY Patented Aug. 23, 1949 RECEIVING SYSTEM FOR CODE SIGrNALS 5 Benjamin S. Vilkomerson, Camden, N. J., assignorto Radio Corporation of America, a corporation of Delaware Applicannianuary 11, 1945, serial No. 572,403

5 claims. (o1. irs- 66) This invention relates to receiving systems for code signals and more particularly to apparatus for reducing the effects of interference when receiving code signals by radio.

In order to reduce interference effects, such as random noise peaks, key clicks of stations on adjacent channels, and other adverse eiects, it is important that the received signals should be regenerated before application to a telegraph signal recorder or printer. This is preferably done in such a Way that a substantially rectangular wavev is produced representing the marking and spacing elements of the signals. Various limiting devices are known in the art which aid in accomplishing this purpose. Insoar as I know, however, it is a novel technique to use in combination an amplitude limiter and a time integrator to discriminate against pulses of shorter duration than the signaling dots which are used in the composition of the code signals. Accordingly, it is an object of my invention to provide a translating circuit arrangement for suppressing the effects of short pulses having their origin in noise and other interference efiects and to feed the wanted pulses of dot and dash duration directing to the. responsive device which may be used for relaying, recording or printing the intelligence.

It is another object of my invention to provide an adjustable resistance-capacitance integrator stage following an amplitude limiter in a receiving circuit for code signals, this integrator being so adjusted that pulses of the duration of adot vat the keying speed in use will build up sufficient voltage on the grid of an amplifier tube so as to unlock this stage, allowing locally generated tone voltage to go through, while pulses of shorter duration than the duration of a` dot, such as noise peaks r key clicks, since they can.- not be of greater amplitude than the signals, due to limiter action, do not build up suflicient voltage to open the audio channel.

. My invention will now be described in more detail, reference being made to the accompanying drawing, the sole figure of which shows diagrammatically a preferred circuit arrangement.

Referring to the drawing, I show therein a radio receiver I, the intermediate frequency output from which may be fed to a diode rectifier tube 2, wherein rectication of the intermediate frequency signals is obtained. The last I. Fi amplier transformerof the lreceiver is connected to anode 4 of diode rectifier tube 2 by lead wire 6, and by means of condenser 1 to cathodeV which is grounded. (This diode is usually inside the receiver.) The rectified code signals which appear as negative voltages to ground across condenser 1 are conducted by lead 3 to resistor I2 through which the D. C. circuit to ground is completed.

The rectied signal potentials are caused to control one of the grids I3 in a D. C..amplier tube Ill which is of the twin triode high Inu type. The electronic path shown in the left section of this tube has an input circuit which includes resistors II and I2 leading from the grid I3 to ground. This section of the tube is normally maintained in a saturated state, but is biased to plate current cut-oli by a rectied signal potential of sufficient negative value applied to the grid I3. The cathode I4 is, therefore, maintained normally at such a value of negative bias with respect to the grid I3 as to ensure operation at signal amplitudes, and not by weaker spurious pulses, this being accomplished by proper setting of a movable tap on a potentiometer I5 in the circuit between a source of negative voltage 4I) and ground.` One end of the potentiometer I5 is connected to a tap on a second potentiometer I5a which is connected between ground and the negative terminal of the source 40. The use of two potentiometers' I5 and I5a is found advantageous for fine and coarse setting of the threshold bias.

The right hand section of tube Il! contains an independent cathode I6, a control grid Il, and an anode I8b. The grid II is connected by a resistor I9 to the anode I8a in the left hand section of tube II). Anode potential is fed from source @El through resistors 28, 25, and 20 to the anode i3d in the left section of the tube IB. Resistor 20, however, is common to the output circuit of the left hand section and the input circuit of the right hand section. The junction between resistors 20 and 25 is connected to ground through a potentiometer 2I,to a tap on which cathode I6 is connected. A resistor 26 also connects the junction between the resistors 25 and 38 to ground.

The input circuit for tube 23 includes a lter network consisting of resistor 22 and a selected one of a number of capacitors 3l of diierent ratings which maybe adjustably inserted in circuit between resistor 22 and the cathode 24. A selector switch 3 is used to determine which of the capacitors 3I shall be used in this lter. If desired, the switch 30 may be so constructed as to insert one or more of the capacitors 3! in the circuit. The choice of these capacitors SI is determined by the keyingspeed, by the operating characteristics of the circuit as a whole and by the prevailing character of the noise effects which are to be eliminated.

In operation, the threshold bias potentiometers I5 and I5a. are adjusted according to the signal strength, so that the cathode I4 is somewhat less negative in respect to ground lthan the grid I3 as biased by rectied signals.` This means that interfering signals-,'or noise ofr aA grid voltage which has less amplitude than the desired signal, will result in lower negative relative to ground than the cathode so that the gridY I3 remains positive in respect to the cathode, and the voltage on anode I8a remains lowabout +10 volts. It should be noted that resistor II' which is of the order of 1 megohm is in series with the grid I3 and limits the now of grid current to. ra few micro-amperes as long as the grid is positive by any ordinary amount. Very nearly all of the positive voltage applied appears across this resistor, so that the grid actually goes only very slightly positive with any applied positive voltage. When the gridy I3 goesv negative, due to rectified signal voltages. of larger negative magnitude than the negative bias on cathode Illl being applied, the grid: current stopsand, there being no. voltage drop across resistor II, the full negative voltage is applied to. the grid I3. This reduces or cuts ofi' the current flowing in circuit with anode Ia. The anodevoltage rises to a more' positive value dueto a reduction of the voltage drop across. resistor 20. Cathode I6 having been set to such a positivevoltage by: potentiometer' 2l as to make grid I'Ii` negative in respect to it in the absence of signal, whenzanode [8a goes-more positive, due to a signal: being impressed, grid Il goes positive also (limited by resistor i0), thereby reducing the positive voltage on anode Ib and on grid 4I of tube 23; This increases the positive voltage on anode 2l which, being connected through resistors. 33 to the grids of tube 32, changes the potential of these grids in respect to their cathodesfrom a negative value corresponding to plate current cut-ottv to a normall amplifier bia-s; The locally generated: tone frequency voltage applied to the grids of tube32 is thus stopped or passed onftotransformer lin accordance with the presence or absence of signal voltage on grid I3. The sensitivity of the device is such that complete switching, on and oi of the local tone source can be accomplished-with a diiierential voltage of. as little as 0.2 volt on grid i3 above the adjustable threshold bias voltage on cathode it; Any value of. signal above this, upto '75 volts or-more, has exactly'tliesame effect on the voltage of anode Ib, so: that theeiect of tube Iilis'toinake all signals that pass through it have; exactly the same amplitude. Being allor thesame amplitude the voltage impressed on gridv 4i will bea function of the time during which current flows through resistor 22 to charge condenser 3|; By proper selection of values, pulses of shorter` dura-tion thanthe dot of the keying speed in use will not build up sucient voltage to-cause tubeA 23 to switch on the local tone voltage. In this way noise pulses of short duration can Vbe completely eliminatedirom. the output of transformer 31 even when their originalamplitude (on grid I3) is many times greater than that of the desired signal. I

The setting of switchfshould be such asto eliminate .pulsesvk of shorter duration than any which are used as-signaling dots at a given keying speed. In an embodiment-of my inventionwhich has. been. constructed and.l successfully: operated,

there were eight different condensers 3| connected to diierent points on the selector switch 30, and the values of the respective condensers were as follows:

M. F. D.

The output circuit of the D. C. amplifier 23 includes resistor 29 serving as a load. Utilization, however, is` obtained by connection of the anode 27 to the grids in a push-pull twin triode amplier tube 32. Grid resistors 33 are used in this connection.

A tone signal from a local source 34 is applied in a push-pull manner to the grids of tube 32through a filter network 35. The two cathodes in tube 32 are interconnected by a potentiometer 3% having a tap connected to ground through resistorzt. The tap is movable in order to equalize the positive and negative peaks in tube 32. Anode potentialA is applied to the anodes in tube S21 by connectionv from the direct current source i0 to a niidtap on the primary winding of the output transformer 3l. The secondary winding of this transformer is suitably connected'A to terminals 35 ci a line which may be used, if desired, for transmission of signals to a commercial receiving station.

In utilizing my invention it will be apparent that the characteristic of the received signals is usually one wherein thev intelligence signals are accompanied byfunwanted interference effects. The operation of the limiting devices including tubes l (land 2Sin combination with the lter system composed ofy resistor 22and a selected one of the condensers 3l is such as to remove substantially the greater part of the interference eiects and to 'deliver a noise-free signal wave.

The choice'of suitable values for the resistances and capacitances of the network depends on keying speed only and canY bev and were calculated to accommodate the speeds in use.l The lowest speed naturally gives the highest amount of noise rejection, as noise pulses of longer duration can be rejected without losing dots. The combination of resistive and capacitive components covers a range of time constants of from .0004 second to 05 second when using a r-esistor 22 having a value of'470,000 ohms and when using the capacitive values given in the above table. This is roughly equivalent to dots of keying speeds of from 20 words per minute-to the highest speeds used with automatic transmitters.

It will, of course, be understood bythose skilled in the art that variousfmodi'cations of the `invention may be made without departing from the spirit thereof'. I do not wishl to be restricted, therefore, to the precisev circuit arrangement illustratively` shown herein, nor do I feel restrictedto thechoice of components-having the values illustratively stated.

I claim:

`1. In a -signalreceiving system', a multi-stage amplifier including at least one stage whichv is biasedY to be un-responsivetoinput voltages of lessthan an adjustable predetermined amplitude, and to passV on to succeeding stages voltage pulses of constant amplitude wheneverthe said input voltage exceeds, by either a relatively small or a large amount, the aforesaid predetermined amplitude, said one stage being followed by a first amplifier stage having an input circuit which includes resistance and capacitance, means for adjusting the value of said capacitance so as to integrate pulses of energy derived from the said one stage and thereby to differentiate between such pulses as constitute intelligence signals and pulses caused by random noise peaks, key clicks and the like, a local source of low frequency energy, and a second amplier stage having an input circuit which is subject to joint control by said local source and by the output from said first ampliiier stage, said second amplifier being thus rendered responsive only to signal pulses representing said intelligence signals.

2. A system in accordance with claim 1 in which said means for adjusting the value of the capacitance includes a multi-position switch having selective contacts each connectable to a respective one of a plurality of differently rated capacitors.

3. A signal translating arrangement comprising two amplifier stages biased to be un-responsive to input voltages below an adjustable threshold value which amplifier stages are operated to produce voltage pulses of equal constant amplitude for input signals of all amplitudes above the threshold value, followed by a resistance capacitance integrator stage and a push-pull ampliiier stage in cascade in the order recited, means for constantly applying tone frequency energy to be keyed on and off in accordance with the marking and spacing conditions of an incoming train of code signals to said last amplifier stage, said first amplier stages being operative to deliver substantially rectangular waves to said integrator stage, said integrator stage possessing a time constant circuit the parameters of which are so chosen as to nullify the effects of random noise peaks, key clicks and similar interference the persistence of which is of shorter duration than that of a signaling dot, and said integrator stage having its output circuit coupled to the input side of said tone frequency amplier stage, whereby the keying of said tone frequency energy is effected.

4. A receiving circuit for discriminating between intelligence signals and noise signals, comprising direct current amplifier means for leveling the signal wave peaks to a limiting value, a direct current amplifier stage having an input coupled to said amplifier means and having output electrodes, said last stage having a device in its input circuit for integrating energy pulses which constitute signal components derived from said limiting amplifier means, such device being effective to dissipate the energy components of noise signals whenever they persist for a shorter time than the duration of a signaling dot, another amplifler having a tone frequency source connected to its input circuit, and means for keying the output of this amplifier by the signal energy derived from said energy-integrating amplifier.

5. A circuit arrangement for the reception and translation of code signals comprising a plurality of cascaded amplifier stages, at least one of said stages being operable as a minimum input and maximum output amplitude limiter, a following stage having an energy integrating device associated with its input circuit, said device including a time constant circuit at least one of the components of which is adjustable to produce discrimination between the signal dots of code reception at a predetermined keying speed and noise pulses of shorter duration than said signal dots, said discrimination being thus effective to cause the output of said following stage to be substantially noise free, a source of tone frequency energy, a push-pull amplifier stage having its input circuit subjected to push-pull control by said source, and means under control of code signal potentials in the output of said following stage for keying said push-pull amplifier stage on and oil?.

BENJAMIN S. VILKOMERSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,916,404 Barton July 4, 1933 2,061,734 Kell Nov. 24, 1936 2,153,202 Nichols Apr. 4, 1930 2,173,154 Bernard Sept. 19, 1939 2,250,550 Peterson July 29, 1941 2,262,838 Deloraine et a1. Nov. 18, 1941 

